Pharmacological Analysis of Sterol Δ8-Δ7 Isomerase Proteins with [3H]Ifenprodil

Sterol Delta8-Delta7 isomerases (SIs) catalyze the shift of the double bond from C8-9 to C7-8 in the B-ring of sterols. Surprisingly, the isoenzymes in fungi (ERG2p) and vertebrates [emopamil binding protein (EBP)] are structurally completely unrelated, whereas the sigma1 receptor, a mammalian protein of unknown function, bears significant similarity with the yeast ERG2p. Here, we compare the drug binding properties of SIs and related proteins with [3H]ifenprodil as a common high affinity radioligand (Kd = 1.4-19 nM), demonstrating an intimate pharmacological relationship among ERG2p, sigma1 receptor, and EBP. This renders SIs a remarkable example for structurally diverse enzymes with similar pharmacological profiles and the propensity to bind drugs from different chemical groups with high affinity. We identified a variety of experimental drugs with nanomolar affinity for the human EBP (Ki = 0.5-14 nM) such as MDL28815, AY9944, triparanol, and U18666A. These compounds, as well as the fungicide tridemorph and the clinically used drugs tamoxifen, clomiphene, amiodarone, and opipramol, inhibit the in vitro activity of the recombinant human EBP (IC50 = 0.015-54 microM). The high affinity of the human EBP for 3H-tamoxifen (Kd = 3 +/- 2 nM) implies that the EBP carries the previously described microsomal antiestrogen binding site. Interactions of the EBP with structurally diverse lipophilic amines suggest that novel compounds of related structure should be counterscreened for inhibition of the enzyme to avoid interference with sterol Delta8-Delta7 isomerization.